Anti-reflective (AR) coatings are used in several industries, including in the manufacture of photovoltaic (PV) modules, to reduce the reflection fraction of incident light as light passes through an optically transparent element such as glass. The goal of AR coatings for glass substrates is to achieve a refractive index that is as close to 1.23 as possible to maximize light transmission over a broad band of light wavelengths.
One or more layers of a low refractive index coating can achieve improved transmittance in a broad wavelength range and a wide range of incident angles. Such coatings can be deposited (at atmospheric pressure or without vacuum) via sol-gel processes and can be highly cost-effective. These thin anti-reflective coatings, which may be formed from a silicon dioxide precursor applied to the glass by conventional coating techniques, have been reported to improve solar light transmittance by about two to three percent in the visible portion of the light spectrum. Such sol-gel materials have been formed using several mechanisms including via hydrolysis/condensation reaction of alkoxysilanes. See, e.g., G. Wu et al., “A novel route to control refractive index of sol-gel derived nanoporous films used as broadband antireflective coatings,” Materials Science and Engineering B78 (2000), pp. 135-139.
Sol-gel coatings can be applied to optically transparent elements using a variety of methods, including spin-on, slot die, spray, dip, curtain, roller and other coating techniques. The total amount of solids as expressed in terms of oxides included in the coating formulation may be varied from about 1 to about 25 weight percent depending upon the application method. Conventional solvent systems used in sol gel processes generally have low boiling points and high volatility so that they evaporate rapidly after the coating is applied to the substrate in order to minimize cure time and temperature conditions.
One challenge with certain wet coating application methods, and in particular certain continuous coating methods, is that it is difficult to achieve a uniform coating on the optically transparent substrate(s). The use of sol-gel coating formulations comprising conventional low boiling solvent systems tend to exhibit an increased concentration of solids and/or viscosity over time resulting in a non-uniform coating. This can lead to quality assurance challenged as well as increased material costs.